How to set up min/max/default value for node group socket via Python?
You can access it through the NodeGroup object's inputs:

glossinessFactorInput = bpy.data.node_groups["NodeGroup"].inputs['Glossiness Factor']

glossinessFactorInput.min_value     = -5.0
glossinessFactorInput.max_value     = 5.0
glossinessFactorInput.default_value = 1.5